To evaluate Angular2, I started to convert a TypeScript application hosted in an ASP.Net MVC application to be hosted by the Angular framework.

The application continuously polls data from a web service, performs a couple of calculations, and displays the calculated results. It is a display-only auto-paging application – the only user interaction being to allow for manual paging. Parts of the display are updated by requesting a PartialView and adding it to the display using jQuery append().

create application directory and configuration files, and run npm install

Components

First, we need to translate MVC Controllers and Views into Angular2 @Component classes and templates:

A page (controller+view) typically translates into a component with an associated HTML template.

Depending on your application, a PartialView may also be represented by a component. It may also translate into a part of a template being activated by NgSwitch or NgIf.

The file app/app.component.ts defines the URL routes, with @RouteConfig being the equivalent of MVC routing configuration. Each page you can directly address by a URL in a browser needs to be added to the @RouteConfig declaration.

Angular Exceptions

Currently, the only reliable way for me to resolve the condition is to close the browser window, restart npm lite-server, and hitting ctrl-F5.

Unfortunately, this was not the only (recurring) exception generated by the Angular2 framework. True, this is still a beta version (15), but if an exception in the framework occurs, there is almost no way of debugging it, as developers are used to debug their code. There is simply too much magic happening in the background. I repeatedly had to remove code or html and re-insert it step by step to figure out which part of the code caused a framework exception. And judging from browsing through GitHub issues, the situation has improved considerably from earlier versions.

Coding and Legacy Code

If your component needs to process URL parameters, there is the built-in routeParams.get() method. There is no automatic mapping of URL parameters to model class properties, but I developed a simple TypeScript function which will provide this convenience.

Adding and referencing existing TypeScript code to an Angular2 application is straight-forward. As I wanted to keep changes to existing code to a minimum, I simply added the scripts in the main index.html page using <script> tags.

However I found that Angular2 does not render <script> elements inside HTML templates. Fortunately, this code can be moved to and thus accessed from the @Component class, either in its constructor or one of the component’s lifecycle hooks, the Angular equivalent of ASP.Net events.

Angular2 uses import to load and reference TypeScript code. To reference code loaded by a <script> tag, you need to declare external symbols, as is typical for TypeScript’s .d.ts files.

To implement calls from the legacy code into the Angular2 code, I needed to store the Angular component in a window property, and access this property to invoke a method of that component:

Just a tiny change. Alternatively, every referenced declaration (and their dependencies) would have needed to be marked as export.

Comparing Architecture and TypeScript Hosting in ASP.Net MVC and Angular2

Dynamic Views

To dynamically render additional views, I created another component and assigned it a selector. The parameters displayed in the dynamic view are stored in a class, and the main component stores an array of this class:

Disabling some Angular2 features

Angular2 allows CSS styles per @Component, and when rendering a component’s CSS, adds a component-specific attribute to the HTML elements, as well as to their CSS declarations.

Since I did not want to edit my CSS (which would have certainly made the CSS declarations cleaner and more specific), I deactivated this CSS magic by declaring the component

encapsulation: ViewEncapsulation.None,

Also, I wanted to disable Angular’s data-binding mechanism, since my legacy code takes care of all data-binding, and I wanted to avoid any interference between the two.

Adding the component declaration

changeDetection: ChangeDetectionStrategy.OnPush

switches change detection and auto-updates off.

Conclusion

Starting with Angular2 certainly comes with a learning curve. Switching platforms, even though they are comparable, may be a major effort. Whereas my little sample just concerned hosting existing functionality in Angular2, adapting existing code as a “real” Angular2 application – designing and using components, CSS handling, data-binding and change detection – is probably an effort similar to writing the application from scratch.

Migrating the code to Angular2, the page’s component, let’s call it ParentComponent, contains an array of ChildComponents. If a new ChildComponent is added to the array, Angular2 will automatically render its template or templateURL, and add the resulting HTML inside the <childcomponent> tag

Your custom JavaScript code can then process the rendered component in one of the LifeCycle Hooks.

The story gets more complicated if your code needs to dynamically select which PartialView to render. Of course, there is the NgSwitch directive, which acts as the templating’s switch/case mechanism, but template code may easily get ugly and overly complex.

Note that I need ChangeDetectorRef to explicitly update the template because I deactivated change detection using ChangeDetectionStrategy.OnPush, so this need not apply to your case.

The essential part is that you need to copy all data referenced by the template to component.instance. #newItem is the anchor of the HTML element in the @Component’s template or template, and the loaded template will be attached after the original template, not replacing the original template.

Whereas in ASP.Net MVC we have a strict distinction between Controller and View, and process and data flow are from the controller to the view, and the view cannot (usually does not) reference the controller, this distinction does not apply in Angular2, where we end up in the component after the template has been rendered.

If you need to pass more than just a couple of values from your Angular2 component to the template, you’ll sooner or later run into the error message

EXCEPTION: Does not support more than 9 expressions

I needed to do just that to provide TypeScript/JavaScript objects with a huge set of values. The solution to work around this restriction is to create an object in the component which stores all these values, and then access its JSONified value

In ASP.Net MVC we can define a parameter model class, which is automatically filled by the framework before invocation of the controller’s method implementing the action. (More precisely, the Databinder sets the values of the public properties of the object)

Whereas routeParams.params is a simple Object with propery names derived from the query string, URLSearchParams contains a Map<string, string[]> (see url_search_params.d.ts in node_modules\angular2\src\http).

I chose routeParams to continue, because I do not use multiple occurrences of URL parameters.

URL parameters are case-sensitive in Angular2 / TypeScript / JavaScript, whereas they are case-insensitive in ASP.Net MVC. To copy the query string parameter values into a model class, a method to ignore case of both the URL parameters and the model class properties is therefore necessary.

To get the names of the parameters, we can use the function Object.keys() applied on routeParams.params, and we map the lower-case version of the name to the name.

Next, we iterate through the Object.keys of the parameter model class, and assign the model class property the value given by the query string parameter if one exists: